During any payout application of optical fibers, substantial signal losses can occur when the length of the fiber is turned about a relatively small diameter of curvature (macrobending) or when very small bends (microbending) are introduced. The severity of these losses depends on the magnitude of the bending introduced, such as the sharpness of the curvature of the macrobend, or the number of microbends along the length of the fiber.
Bendloss is a function of the bend diameter as well as of the wavelength of the light propagating through the fiber. For modem single mode type fibers, low levels of attenuation allow the use of extremely long lengths of optical fiber without intermediate repeaters or amplifiers, provided certain physical constraints are considered.
Bendloss consists of two components: a transition loss and a pure bend loss. The transition loss occurs when a change in curvature of the fiber axis is such that coupling of light changes from the fundamental mode to leaky core mode. The pure bend loss results from the continual loss of guidance at the outer portion of the evanescent field of the fundamental mode. This ejected radiation from the core can set up what is called xe2x80x9cwhispering galleryxe2x80x9d (WG) modes in the cladding in which the leaking light can be re-coupled back into the primary core conducting mode.
Current fiber bendloss test equipment used by fiber manufacturers measures fiber optic attenuation at one constant bend diameter, over one constant bend angle and at one constant tension. To do so, the fiber is simply wrapped a number of times around a mandrel of a specific given diameter under a given tension. Then a comparison is made between the power input to the fiber and the power output from the fiber and the attenuation of the fiber calculated from this comparision data. As is obvious, the bend sensitivity information thus produced is severely limited in its usefulness, being helpful only in applications where the bend radius of the fiber is equal to or greater than that used during the test. More importantly, the use of relatively long lengths of fiber wrapped many times about a mandrel, for all practical purposes, eliminates any effects due to WG modes.
For typical payout fibers of short lengths, bend diameters of less than 3 mm enable conditions that allow WG modes to originate within the cladding. The interference between these WG modes and the core fundamental mode causes oscillatory power loss in the core. To evaluate candidate optical fibers for diverse payout applications, it is greatly desirable to have a means to measure the bendloss characteristics of the optical fibers under realistic conditions involving various degrees of tension applied to the fibers as well as various bend angles and bend diameters. One such realistic application is the use of optical fibers in Fiber Optic Guided Vehicle (FOG-V) dispensers where information concerning the magnitude of optical attenuation during fiber deployment is critical.
The Dynamic Bendloss Measuring Device (DBMD) allows the determination of the bend sensitivity of a single-mode optical fiber by subjecting it to dynamically changing bend angles at varying degrees of tension and bend diameters. It utilizes a swing arm capable of sweeping an arc subtending angles ranging from 0xc2x0 to 170xc2x0 at any given bend diameter and fiber tension and takes the bendloss measurement at each degree of the range. The varying bend diameters are provided by pins of diverse diameters and can be easily effected by substituting one pin for another while the variation in the applied tension can be effected by changing the input current setting in the tension assembly. With each new pin and tension setting, the swing arm sweeps through the pre-selected range of bend angles enabling the calculation of the bendloss at any of the bend angles.